In the prior art method illustrated by equation (1), the reaction water dilutes the acid and inhibits the oxidation, which cannot be considered as a satisfactory synthesis from an industrial viewpoint when the water content exceeds 30%, and sometimes even only 20% by weight of the reacting mixture. Besides the necessity to keep the acetic acid anhydrous there is the necessity, in this type of synthesis, to recover the acetic acid from the more or less dilute solutions which are originated in various parts of the plant, as for example the mother liquor coming from the centrifuges where the solid terephtalic acid is separated. Until now the different aqueous-acetic solutions were fed to a rectification column, where a great number of trays and a high reflux ratio allowed to obtain almost anhydrous acetic acid (on the column bottom) and water containing acetic residues (at the column top). The distillations performed so far, are not free from drawbacks; in fact they involve:
(a) a great number of trays (in some cases up to 80) and an excessive steam consumption, in order to obtain, at the column top, a water stream containing no more than from 1000 to 5000 ppm of acetic acid and, on the column bottom, an anhydrified acid stream containing no more than 3% by weight of water; PA0 (b) the total loss of the methyl-acetate leaving the column top along with water. PA0 (a) the liquid obtained by condensing the released vapours and said mother liquor are fed both to an azeotropic distillation system in which the azeotroping agent is isobutyl acetate; PA0 (b) the light ends of the azeotropic distillation, richer in water, are condensed and demixed into two phases, wherein the aqueous phase, which is lower and contains isobutyl-acetate and methyl-acetate, is conveyed to a stripping, whereby isobutyl-acetate and methyl-acetate are recovered, and wherein the organic phase is recycled to the azeotropic distillation.